This is a proposal to develop of an intraoperative imaging probe that can assist the surgeon, especially the neurosurgeon, in determining the location of residual tumor during surgery. In its intended application the patient is injected with a tumor seeking radiopharmaceutical prior to surgery, during surgery the body of the tumor is removed, and the residual tumor is located using either a gamma- or beta sensitive imaging probe. The gamma-sensitive imaging probe would be employed for isotopes such as, 99mTc, 1231, 1251 or 201Tl, which emit low energy gamma-rays or x-rays. Since the small size of the probe would preclude adequate shielding for 511-keV photons, the positron itself would be detected with a beta sensitive / gamma-ray insensitive imaging probe in those cases in which positron emitting radiopharmaceuticals are being employed. The probe is an array of plastic scintillating fibers, a thin sheet of CaF2(Eu) or a thin sheet of plastic scintillator, for beta imaging, or an array of CsI(Tl) crystals or a single thin NaI(Tl) crystal for gamma imaging. Each is coupled to a bundle of optical fibers, which are in turn coupled a position sensitive photon detector. Two photon detector technologies are to be investigated for this purpose. The first technology, referred to as a Visible Light Photon Counter (VLPC), is a recently declassified device, which is still classified in its infrared sensitive version. The VLPC has an 80% quantum efficiency, and operating at 8 deg K, it has essentially no noise and is capable of counting single photons. The second technology is the position sensitive photomultiplier operating in a multianode or crossed wire configuration to maximize positioning accuracy. The detector array and fiber optic bundle, which can be detachable subunit, can isolate the probe from the electronics and provide a sterilizable unit appropriate for the operating room. The detachable imaging probe, itself, can be relatively inexpensive and can be produced in a variety of sizes and shapes to fit the specific problems of each surgery.